This invention relates, in general, to composites of polymers reinforced with finely divided solid particles (filler). The polymer component is derived from a prepolymer, for example a hydroxy-terminated (OH-terminated) prepolymer, cured with a curing agent, in this case, a polyfunctional isocyanate. The prepolymer may be combined with a plasticizing agent (plasticizer). In one embodiment, the filler may be inert particles such as clay, calcium carbonate or glass beads, as for example in synthetic concrete, or an abrasive particle such as silicon carbide in a grinding wheel. In another embodiment, the filler may be oxidizer and/or fuel for a solid propellant, in particular, a composite such as is formed within a confined space, for example the casing of a rocket, upon curing of a binder matrix or binder system. Such composites are referred to as propellant composites in which particles of a solid propellant are held together by a cured submix, the submix comprising a plasticizer, the prepolymer and modifiers such as burning rate additives, wetting agents, and the like. The binder matrix is formed by curing the submix, after the filler is added, with the curing agent, optionally with the addition of a curative catalyst.
The solid particles in the propellant typically include an oxidizing agent (oxidizer) such as cyclotetramethylenetetranitramine (HMX) and/or cyclotrimethylenetrinitramine (RDX), or other nitramine (N--NO.sub.2) group containing crystals, together referred to herein as `nitramines`, other oxidizers such as ammonium perchlorate, ammonium nitrate and the like, as well as fuel particles of boron, aluminum, metal and organic hydrides, and the like.
Composites of this invention are said to be filler-reinforced because of the strength they derive from the use of a neutral polymeric bonding agent which is thought to form hard and tough shells around the filler particles. With sufficiently good bonding, extensive dilatation upon extension of the propellant is prevented.
In the prior art, a strong interfacial bonding in the composite is obtained by mixing a bonding agent into a premix slurry during processing in a non-polar polybutadiene-based and polypropylene glycol based propellants. Examples of typical bonding agents for hydroxy-terminated polybutadiene (HTPB) and polypropylene glycol (PPG) are basic amine oligomers, small molecules such as alkanolamines, alkanol amides, Dantocol and amine salts. They readily undergo chemisorption or adsorption on polar solid particles since the binder matrix is non-polar, but in a polar binding system where a nitro- or nitration group containing plasticizer is used, the prior bonding agents are ineffective since they are too soluble in the polar submix.
In U.S. Pat. No. 4,410,376, for example, Bruenner et al note that 2,3-dihydroxypropyl-bis(2-cyanoethyl)amine is soluble in the binder phase only to the extent that it can be adsorbed on the filler particles. If too soluble, as it is in a nitroplasticized energetic system, it becomes inefficient. But, other than by trial and error, there has been no suggestion as to how to choose a bonding agent with a high degree of probability that it might be an effective bonding agent.
Given the desirability of depositing the bonding agent from the submix, Bruenner et al disclose the problems of doing so, using a polar binder. They recognized that 2,3-dihydroxypropyl-bis(2-cyanoethyl)amine was not only too soluble, but also that its high basicity degraded the composite. Therefore they partially neutralized the molecule and tried hard to maintain some of the free amino groups to get the chemisorption on the surface of the nitramine particles. It remained a small molecule with a molecular weight (mol wt) no greater than about 1000, having few contact points (or anchoring sites) per molecule, to the surface of a filler particle, and they had no suggestion as to what changes might be made to improve the bonding in a polar system. By "anchoring sites" I refer to the multiplicity of sites on a large flexible molecule, such as a polymer, which are available for adsorption on the surface of a filler particle. A relatively small molecule has few such sites.
It is evident that if Bruenner et al completely neutralized the TEPAN there would be no basic amino groups for the chemisorption which is instrumental in their obtaining effective interfacial bonding.
In a composite in which plasticized urethane rubber serves as the polar binder matrix, the binder matrix is formed by end-linking hydroxy-terminated prepolymers with polyfunctional (di-, tri-, or higher) isocyantes. Curing takes place after the slurry has been cast, usually into a chamber of a rocket motor.
A good bonding agent should (i) be accumulated on the surface of the solid particles of filler, (ii) undergo a crosslinking reaction with a curing agent to form the hard and tough shell around the solid particles, and (iii) have enough functional groups left over to form primary bonds between the shells and the binder matrix.
When successfully executed, the "bonding agent method" of this invention is effective without pre-coating the filler, and is economically advantageous over any method comprising pretreating or precoating the solids.
In the prior art, in a polar binder matrix, bonding agents provide interfacial bonding only when they are used to precoat the filler so as to form hard and tough shells enveloping the filler particles. The result is "filler reinforcement" which increases strength and stiffness of a composite by interfacial bonding between the filler particles and the generally elastomeric matrix. Nonreinforcing fillers do not substantially increase the tensile strength because there is little interfacial bonding, thus suffer dewetting upon deformation.
The choice of plasticizer is not narrowly critical provided that it is polar, and it may be a 1/1 mixture of bis(dinitropropyl)formal and bis(dinitropropyl)acetal (BDNPF/A), bis(fluorodinitroethyl)formal (FEFO), nitratoesters including but not limited to nitroglycerin (NG), metrioltrinitrate (METN), trimethyloltrinitrate (TMETN), butanetrioltrinitrate (BTTN), diethyleneglycoldinitrate (DEGDN), and triethyleneglycoldinitrate (TEGDN), or any combination of the foregoing, inter alia.
This invention specifically relates to polyurethane composites in which the solid particles are not precoated prior to mixing the components of the composite. Instead, the bonding agent is added during processing to yield interfacial bonding.
More specifically, this invention relates to a neutral polymeric bonding agent (NPBA) which nevertheless is adsorbed on the surface of polar filler particles from a polar submix. In general, a prior art bonding agent for a non-polar binder such as OH-terminated polybutadiene, is ineffective in my system. For example, a polar small molecule (less than about 1000 mol wt) bonding agent is ineffective because my binder is polar and these small molecules remain in the submix; small molecules do not have a sufficiently large number of anchoring sites per molecule to be effectively adsorbed. Also, prior art basic oligomer bonding agents such as TEPAN and partially neutralized TEPAN, are likely to cause poor cure and/or degradation of energetic polymers and plasticizers. By "energetic" I refer to materials containing nitro-, nitrato- and/or azido polar groups.
This invention is specifically concerned with nitramine crystal/polar binder systems for which my NPBA is found to be most effective. By "neutral" I refer to a binder free of amine and acid groups (but not a salt), having a pH in the range from pH 5.5 to about pH 8.5, and more preferably from pH 6 to pH 8. In the past, the search for the most effective bonding agent was heretofore carried out by trial and error, by making and testing batch after batch of propellant with a wide variety of potential bonding agents including nitrocellulose (NC). Though neutral and polymeric, NC is either too soluble and does not undergo phase separation at the slurry processing temperature, or is insoluble in the submix, and therefore, in either case, is ineffective. The slurry processing (or `slurry mix`) temperature is a predetermined temperature in a narrowly defined range, typically a range less than 5.degree. C. (9.degree. F.), for a selected submix. The criteria for solubility of the NPBA in the submix are described hereinbelow.